The present invention relates to a position signal demodulating method and a position sensitivity determining method for a filing apparatus and, more particularly, to a position signal demodulating method for generating a head position signal by using two triangular signals PosN and PosQ which have a phase difference of a 1/4 period and which are obtained by reading the head positioning signal pattern recorded on a medium in advance, and a position sensitivity determining method for correcting the signals PosN and PosQ.
In a filing apparatus such as an optical disk apparatus and a magnetic disk apparatus, data is read/written by positioning a head at a predetermined position on a disk. FIG. 21 shows the structure of a magnetic disk apparatus. In FIG. 21, the reference numeral 11 represents a cover, and 12 a base. A predetermined number of magnetic disks 14, which are recording media, are attached to a spindle mechanism 13 in parallel with each other at regular intervals, and an actuator assembly 16 which can be freely rotated by a rotary shaft 15 is provided in the vicinity of the magnetic disk 14. The actuator assembly 16 is composed of a driving portion (actuator) 17 on one side of the rotary shaft 15 and a carriage arm 18 on the other side. The driving portion 17 is provided with a voice coil 19 which constitutes a voice coil motor. The same number of carriage arms 18 as the number of the magnetic disks 14 are provided, and a magnetic head assembly 21 is attached to one side or both sides of an adapter plate provided at the end of the carriage arm 18. The carriage arm 18 positions a magnetic head 22 at a predetermined position in the radial direction of the magnetic disk 14.
A multiplicity of tracks are formed on the surface of the magnetic disk 14, and each track is divided into a plurality of sectors. Each sector is provided with a servo area SVA and a data area DTA, as shown in FIG. 22, and a sector mark (servo mark) SM, a track number TNO, and a position information pattern PPT are recorded in the servo area SVA. When a position information pattern signal (head output) read by the magnetic head 22 which moves in the radial direction is passed through a demodulator circuit, two triangular position signals PosN and PosQ which alternate in the transverse direction of the track and which are spaced at .pi./2 (1/2 of the track width, which corresponds to a 1/4 period on the assumption that one period is two tracks) phase intervals are obtained, as indicated by the solid lines in FIG. 23A. It is also possible to produce a sawtooth position signal (offset signal indicating the deviation from the center of the track) Pa by sequentially selecting the position signals PosN and PosQ and the inverted position signals *PosN and *PosQ, as shown in FIG. 23B.
FIG. 24 shows a servo circuit for positioning the head at a target position. In FIG. 24, the reference numeral 13 represents a spindle motor, 14 a magnetic disk rotated by the spindle motor 13, 19 a rotary voice coil motor (VCM) for moving the magnetic head in the radial direction, and 22 a magnetic head for reading/writing data.
The reference numeral 23 denotes an AGC circuit for automatically controlling the gain of a signal read by the head 22 so that the level is constant. The reference numeral 24 represents a position signal demodulator which demodulates the position information pattern signals read by the magnetic head 22 so as to output the two position signals PosN, PosQ which have a phase difference of .pi./2 with respect to each other. The reference numeral 25 denotes an AD converter for converting the position signals PosN, PosQ output from the demodulator 24 into digital values, and 26 a microcontroller unit (MCU) for outputting a designated current value for driving the VCM 19 as a result of servo control, which will be described later, a DA converter 27 for converting the digital designated current value into an analog value, and a VCM driving circuit 28. The AGC circuit 23, the position signal demodulator 24 and the AD converter 25 constitute a read/write circuit.
The MCU 26 conducts servo control by the firmware provided therein, and outputs a designated current value for driving the VCM 19. A block diagram of the function of the processing of the firmware is shown in FIG. 24. In FIG. 24, the reference numeral 26a represents a position detector for detecting a position at which the head 22 is currently situated by using the two position signals PosN, PosQ and a track No., 26b a speed detector for detecting the actual speed Va by differentiating the position signals PosN, PosQ, 26c a designated speed generator for outputting a predetermined designated speed Vc on the basis of the number of tracks between the track at which the head 22 is currently situated and the target track, 26d a speed difference calculator for outputting a difference signal Vd indicating the difference between the designated speed Vc and the actual speed Va, 26e a position signal selector for outputting the offset signal (position deviation signal) Pa shown in FIG. 23B by sequentially selecting the position signals PosN and PosQ and the inverted position signals *PosN and *PosQ, and 26f a switching portion for outputting the designated current value which corresponds to the speed difference signal Vd output from the speed difference calculator 26 until the head 22 reaches the target track, and outputting the designated current value which corresponds to the position deviation signal Pa when the head 22 reaches the target track. The reference numeral 26g denotes a position sensitivity determining portion for determining a position sensitivity Sp in a position sensitivity determination mode by using the signals PosN, PosQ. The signals PosN, PosQ are multiplied by the position sensitivity Sp obtained from the position sensitivity determining portion 26g at the time of ordinary positioning control, thereby correcting the amplitudes of the signals PosN, PosQ. It is also possible to input the position sensitivity Sp to the AGC circuit 23 so as to correct the amplitudes of the signals PosN, PosQ.
When the target track is input, the designated speed generator 26c generates the designated speed Vc on the basis of the number of tracks between the track at which the head 22 is currently situated and the target track. The switching portion 26f selects the speed difference signal Vd and outputs it as the designated current value for the voice coil motor 19. The voice coil motor 19 then starts to rotate and moves the head 22 toward the target track at the designated speed. The head reads the position information pattern PPT recorded in the servo area while moving and outputs the signals. The read signals (head outputs) of the position information pattern PPT are input to the position signal demodulator 24, and the position signals PosN, PosQ are demodulated and input to the MCU 26. By using the position signals PosN, PosQ and the track number, the position detector 26a updates the track at which the head 22 is currently situated, the speed detector 26b detects the actual speed of the head 22 and the position signal selector 26e outputs the position deviation signal Pa.
The designated speed generator 26c generates the designated speed Vc again on the basis of the number of tracks between the track at which the head 22 is currently situated and the target track, and the speed difference calculator 26d outputs the difference signal Vd indicating the difference between the designated speed Vc and the actual speed Va. The switching portion 26f selects and outputs the speed difference signal Vd, and thereafter the same operation is repeated, so that the head 22 approaches the target track.
When the head 22 reaches the target track, the switching portion 26f switches the speed control over to the positioning control, selects the position deviation signal Pa output from the position signal selector 26e and outputs it as the designated current value. The designated current value is converted into an analog value and input to the voice coil motor 19. The voice coil motor 19 then rotates so that the head 22 is situated at the center of the track. The head position is controlled by the positioning control based on the position deviation signal Pa and, finally, the head 22 is positioned at the center of the target track. Thereafter, tracking servo control is executed so that the head 22 is situated at the center of the track.
The servo control is continuously executed in the above explanation. Actually, however, the MCU 26 discretely executes the servo control at every predetermined sampling time. To state this concretely, a servo mark SM is recorded in the servo area SVA of each sector, and servo interruption occurs in the MCU 26 every time the servo mark SM is read. When the servo interruption occurs, the MCU 26 fetches the position signals PosN, PosQ and executes the above-described processing on the basis of the position signals PosN, PosQ. In other words, the period of servo interruption constitutes the sampling period, and the discrete servo control is executed at every sampling. The MCU26 calculates a head position signal by adding the track number TN0 read from the servo area SVA to the position deviation signal Pa, and executes a servo control based upon the calculated head position signal.
(a) First Problem
FIG. 25 is an explanatory view of signals for explaining the principle of demodulating head position signals, and FIG. 26 is a decoding table used for decoding head position signals. In FIG. 25, the reference numeral 31 represents a signal PosN, 32 a signal PosQ and 33 a position deviation signal (offset signal) Pa. The abscissa represents a track number, and the ordinate represents a value (-0.5 to 0.5) of an output signal converted into the position from the center (0.0) of the track. For example, if it is assumed that the peak voltage Vp (=.+-.2.0 volt) of the signals PosN and PosQ is converted to the valve 0.5, the value obtained by multiplying the voltage V of PosN, PosQ by 0.5/Vp (=1/4) is represented by the ordinate. A position sensitivity is a gain which makes a peak value which is not 0.5 equal to 0.5. Accordingly, if the peak value is represented by Pk, the position sensitivity Sp is given by the following equation: EQU Pk.cndot.Sp=0.5 EQU Sp=0.5/Pk (1)
In a conventional decoding method, a decoding table such as shown in FIG. 26 is created in advance, and position signals are generated by using the signals PosN, PosQ and a track number in accordance with the program of the MCU. To state this concretely, (i) two consecutive tracks are partitioned into 8 partitions (1) to (8), as shown in FIG. 25, (ii) the signal having the smaller absolute value of the two signals PosN and PosQ is selected in each partition, (iii) the selected signals are connected with each other while the polarities are appropriately changed, thereby generating the sawtooth offset signal Pa which has a rising gradient and which alternates with a period of the track width, and (iv) the track number is added to the offset signal so as to generate a position signal (absolute position signal relative to the reference position). The reason why the signal having the smaller absolute value of the two signals PosN and PosQ is used to generate the offset signal Pa is that when the absolute values of the signals PosN and PosQ become large, the signals become nonlinear due to saturation or the like, so that the relationship between the position and the signals PosN and PosQ becomes nonlinear.
In this manner, a head position signal is conventionally generated by using the decoding table shown in FIG. 26, but this method is adopted on assumption that the track number is correctly read. In other words, misreading of a track number is not taken into consideration in the conventional position signal demodulating method. Since the head reads the position signal pattern and the track number from the servo area SVA of a sector while moving, the head sometimes mistakes the number of the adjacent track for the number of the current track. In this case, since the position signal is wrongly demodulated, correct positioning control and speed control is impossible.
(b) Second Problem
In the read/write circuit, the signals PosN and PosQ are fetched via the AD converter 25, but it is necessary to convert the values of the signals into a unit of a track which is used in the MCU 26. For this purpose, the conversion coefficient (position sensitivity) Sp is obtained and the signals PosN and PosQ are multiplied by the conversion coefficient (position sensitivity). Since the position sensitivity depends upon the core width of the head, there are difference position sensitivities for the respective heads in the same filing apparatus. The position sensitivity also depends upon a change in the gain in the AGC circuit 23, a change in the reference voltage in the AD converter 25, the core width of the read head, and the track width. For example, if the core width of the head becomes small, the position sensitivity also becomes small, and if the track width becomes small, the position sensitivity becomes large.
If the position sensitivity is too large or too small, the angle of the inclination of the signals PosN and PosQ becomes larger or smaller than an ideal angle .theta.i of those signals shown in FIG. 25. For this reason, the connection between the signals PosN and PosQ is not smooth at the time of creation of a head position signal based upon the decoding table in FIG. 26, and there is sometimes a difference in level in the demodulated position signal, as shown in FIG. 27. The broken line in FIG. 27 is an ideal curve of the position signal and (1)-(8) in FIG. 29 indicate the division number for decoding. When the position signal is discontinuous, the positioning control is not uniform, vibration or abnormal sound is produced, and prompt positioning is impossible. To prevent this, several position sensitivity determining methods have heretofore been proposed.
(i) First Method
A first method is a fundamental method in which the position sensitivity is obtained as follows.
(1) The head is positioned at the positions of .+-.0.25 track of each of the even- and odd-numbered tracks, namely, four positions in total (positions a, b, c and d in FIG. 25 at which the absolute values of the signals PosN and PosQ are equal) by the signal PosN.
(2) The absolute values of the signals PosN and PosQ are obtained, and only when the difference between the absolute values is in a preset range, the signal PosN is adopted as the measured value, and the value .vertline.PosN.vertline. is stored.
(3) When the measurement of the absolute values is finished at all the positions, the average value of the measured values is obtained, and the position sensitivity Sp is obtained as the value .vertline.PosN.vertline. at the 0.25 track from the following formula: EQU Sp=0.25/.vertline.PosN.vertline. (2)
Since value .vertline.PosN.vertline. at the 0.25 track position is regarded as the half of a peak value Pk of the signal PosN, .vertline.PosN.vertline.=Pk/2 is obtained. Substituting .vertline.PosN.vertline. in equation (2) with Pk/2, Sp=0.5/Pk, i.e., equation (1) is obtained.
The first method, however, has the following problems.
(1) Since the signal is positioned at the .+-.0.25 track before the position sensitivity is corrected, it is not guaranteed that the positioning at the .+-.0.25 track is accurate.
(2) Since the signal PosN has a gradient in the vicinity of the .+-.0.25 track, it constantly changes.
(3) It is impossible to measure the position sensitivity unless the condition that the absolute values of the signals PosN and PosQ are in a predetermined range is satisfied.
(ii) Second Method
FIG. 28 is an explanatory view of a second position sensitivity determining method. In this method, the position sensitivity is obtained in the following manner.
(1) The voice coil motor VCM is operated at an approximately equal speed.
(2) The values (Y[k-2], Y[k-1]) of the signal PosQ for the latest two samplings are stored in the memory.
(3) The value (Y[k]) of the signal PosN at the point of time at which the signal used for generating a position signal is switched from the signal PosN to the signal PosQ or vice versa is stored in the memory.
(4) The straight line LQ of the signal PosQ is calculated from the values Y[k-2], Y[k-1] of the signal PosQ for the latest two samplings and the straight line LN of the signal PosN which passes the current value Y[k] of the PosN and has the reverse gradient to that of the line LQ is calculated. The intersection of the two straight lines LQ, LN is then obtained.
(5) Thereafter, the position sensitivity Sp is obtained from the following formula: EQU Sp=0.25/.vertline.Vc.vertline.
wherein Vc represents the voltage at the intersection.
The second method, however, has the following problems.
(1) Since the sensitivity is not corrected, the position as the reference for speed control is inaccurate. It is therefore not guaranteed that the speed is an equal speed.
(2) Since the head moves at a certain speed, if the speed is too high, there is a possibility of the signals PosN and PosQ entering a saturation region, which makes the Sp inaccurate.
(iii) Third Method
FIG. 29 is an explanatory view of a third position sensitivity determining method. In this method, the position sensitivity is determined as follows.
(1) The voice coil motor VCM is operated at an approximately equal speed.
(2) The sum .vertline.PosN.vertline.+.vertline.PosQ.vertline. of the absolute values of the signals PosN and PosQ is measured at the point of time at which the signal for generating a position signal is switched between the signals PosN and PosQ. In other words, the sum .vertline.PosN.vertline.+.vertline.PosQ.vertline. of the absolute values of the signals PosN and PosQ when the head is situated at the .+-.0.25 tracks of the even- and odd-numbered tracks is measured. The sum of the absolute values is ideally the constant value (=0.5).
(3) Thereafter, the position sensitivity Sp is obtained from the following formula: EQU Sp=0.5/(.vertline.PosN.vertline.+.vertline.PosQ.vertline.. . . (3).
Since (.vertline.PosN.vertline.+.vertline.PosQ.vertline.) is equal to the peak value Pk of the signals PosN and PosQ, equation (1) is obtained from the equation (3).
This method has similar problems to those of the second method.
As described above, according to the conventional methods, it is impossible to realize accurate position sensitivity correction. Therefore, new logic for determining the position sensitivity is demanded.